There has been a need for a translucent material having a low thermal expansion coefficient, high thermal shock resistance, thermal stability and relatively high impact strength. A product having such a combination of physical properties would be highly desirable for use in windows of wood-burning stoves, for example. It is the object of this invention to provide such a product.
A known class of materials which possess low thermal expansion coefficients, high thermal shock resistance and thermal stability are the glass-ceramics, also known as crystallized glasses. Glass-ceramics are articles in which a major crystalline phase is dispersed in a glassy matrix, at least 50 percent of the bulk of the article usually being comprised of the crystalline phase. The articles are produced from a crystallizable glass and are subsequently subjected to a post-forming heat treatment in order to induce crystallization within the body of the glass. The resulting glass-ceramic articles have lower coefficients of thermal expansion and better resistance to thermal shock than the crystallizable glasses from which they are formed. Glass-ceramic articles may be substantially transparent or opaque, depending upon the particular crystal species produced in the final product. During the crystallization heat treatment, the glass first crystallizes to an intermediate crystalline phase called beta-eucryptite, which is relatively transparent, and thereafter transforms to beta-spodumene, which may be accompanied by opacification. Transformation to the beta-spodumene crystal form improves the strength of an article, but not to the extent desired. Additional information regarding the basic nature of crystallizable glass compositions, glass-ceramics, and methods of making may be found in U.S. Pat. No. 2,920,971 to S. D. Stookey and in U.S. Pat. No. 3,625,718 to R. W. Petticrew.
Various attempts have been made in the prior art to strengthen crystallizable glasses or glass-ceramics. It is well known, for example, that a brittle material such as a glass or glass-ceramic may be strengthened by the introduction of permanent, compressive stresses in its surface layer. For example, glass-ceramics may be tempered by known techniques for vitreous glasses, i.e., thermally or by surface ion exchange. However, the surface compression produced by these techniques tends to dissipate at relatively low temperatures, thereby rendering them unsuitable for strengthening articles intended to be used in high temperature applications.
Another prior art approach to inducing surface compression stresses in crystallizable glass compositions is to differentially crystallize surface portions of the glass article while retaining the interior portion of the article in the glassy state. Variations of this technique are disclosed in the following references: U.S. Pat. No. 3,253,975 (Olcott et al.), U.S. Pat. No. 3,454,386 (Ernsberger), U.S. Pat. No. 3,464,807 (Pressau), and U.S. Pat. No. 3,464,880 (Rinehart). Because the strengthened articles produced by each of these prior art techniques retains the bulk of the article in the glassy state, such articles do not possess the low thermal expansion coefficients and thermal shock resistance required for use in high temperature applications.
U.S. Pat. No. 3,756,798 to F. M. Ernsberger discloses a method for inducing compressive stresses in the surface portion of a glass-ceramic article during the crystallization heat treatment by inducing early crystallization in surface portions of the article by exposure to water vapor which acts as a crystallization catalyst. Because crystallization entails a volume shrinkage, the subsequent crystallization of the interior portion of the article creates a compressive stress in the previously crystallized surface portions. The crystal species at both the surface and interior is beta-eucryptite, although it is also mentioned that an opaque product could be made with beta-spodumene at the surface and beta-eucryptite in the interior. To produce significant strengthening with such a method on a commercial scale, a costly heating chamber capable of supplying superheated steam would be required rather than the conventional kiln.